Metal vapor spectral lamp with mercury and a metal halide at subatmospheric pressure



y 9, 1967 1 J. F. RENDINA 3,319,119

METAL VAPOR SPECTRAL LAMP WITH MERCURY AND A METAL HALIDE AT SUBATMOSPHERIC PRESSURE Filed Oct. 22, 1965 2 Sheets-Sheet 1 SOURCE POTENTIAL FIGQI.

y 1967 J. F. RENDINA I 3,319,119

METAL VAPOR SPECTRAL LAMP WITH MERCURY AND A METAL HALIDE AT SUBATMOSPHERIC PRESSURE Filed Oct. 22, 1965 2 Sheets-Sheet 2 22 v A n n n /1 I Y 1' I L7 r r r r r I VARRABLE R. F. POWER SOURCE INVENTOR. JOHN F. RENDINA United States Patent 9 p This is a continuation-in-part of application- Ser. No.

368,066, filed'May l8, 1964, now abandoned.

This invention relates to light sources suitable for spectroscopic uses and, more particularly, to lamps capable of providing light whose spectral characteristic has relatively sharp atomic spectral emission lines. Particular advantages of the light sources of this invention are their simplicity, stability, high intensity, long life and narrow spectral emission lines. These features render them unusually suitable for use as a light source for atomic absorption spectroscopy.

The basic requirements for lamps suitable as light sources for atomic absorption spectroscopy applications and as calibration standards for any kind of optical spectroscopy include: (1) the source must provide a nearly monochromatic spectral line or lines somewhat separated from all other lines and bands, and of the desired wave length or wave lengths; (2) the spectral line wave length must be of a fixed, repeatable, wave length; (3) the emitted'radiation must have suitable intensity (intrinsic bril liance); (4) the spectral line must be relatively narrow. The term spectral line as used herein refers to the spectroscopic image produced by the light emitted by an atomic transition.

To satisfy the first two requirements, it is generally necessary to use a gaseous discharge lamp where an arc or plasma is created in a vapor which will; when electrically excited to optical emission, produce one or more spectral lines characteristic of the vapor material. In most cases, metals are used as the emitting material, and must first be put into a vapor state. This may be accomplished by thermal evaporation or sputtering. One such lamp uses a hollow cathode electrode. A typical hollow cathode type lamp is described in US. Patent No. 3,089,054 issued May 7, 1963, to A. Walsh et al. As is typical of this type lamp, the hollow cathode insures a luminous discharge within the'hollow cathode which is made of a metal whose spectrum it is desired to produce. The luminous region of the discharge is controlled by adjusting the pressure of the rare gas with which the envelope of the lamp is filled.

While these lamps have found rather wide usage in atomic absorption spectroscopy, they suffer from several disadvantages not the least of which is their relatively high cost. Because they depend on the metal evaporated from the electrodes, their life is limited and their operation somewhat unstable. Their requirement for relatively high purity of the electrode metal makes their manufacture difficult. The physical sizes of the prior art lamp are relatively large because of the volume of filling gas required to support the discharge. Lastly, the lamp intensities are relatively low because of the inherent inefliciency of converting electrical power into emitted radiation and the limitations imposed by energy dissipation of the cathode itself. 1 r

An improved spectral lamp is described by Gilbert H. Reiling of the General Electrical Company in Optical Society of America, 54, No. 4, pp. 532440, April 1964. This lamp provides relatively intense atomic-metal emission lines. Basically the lamp is an arc lamp which uses a combination of metallic iodides and mercury in a 3,319,119 Patented May 9, 1967 ice medium to high pressure vapor arc. The metal iodide has a sufliciently high vapor pressure at the normal arc lamp operating temperatures to maintain it in a vapor state and thereby provides the necessary atomic-metal for electrical excitation and line emission spectra. While this lamp represents an improvement over the earlier high pressure mercury vapor discharge lamps, its spectral emission lines are not as sharp as generally required for spectroscopic purposes. Additionally because of the lamps operating characteristics the spectral lines are subject to pressure shift and pressure broadening with the result that the lines of its spectrum become less sharp, or broader-another deterrent to its use as a spectroscopic light source. This broadening is not symmetrical but is greater in the direction of longer wave length, so that the peak or maximum is shifted toward the red.

It is, therefore, an object of this invention to obviate many of the disadvantages of the prior art spectral light sources.

Another object of this invention is to provide an improved light source having relatively stable spectral lines.

Still another object of this invention is to provide an improved light source having relatively sharp spectral lines.

In accordance with a preferred embodiment of this invention an atomic spectral light source is formed by enclosing a pair of electrodes within a sealed envelope. Prior to scaling, the envelope is filled with a metallic iodide, mercury, and a rare gas. The quantities of the materials employed to fill the envelope are limited such that there is a total envelope vapor pressure when operated of less than one atmosphere. Further in accordance with the invention, a suitable source of potential is connected across the electrodes to establish an arc therebetween. A current limiting means is connected in series with the source of potential to reduce the effects of spectral line broadening and shifting.

In accordance with another embodiment of this invention an atomic spectral light source is formed by using an electrodeless discharge lamp in which prior to sealing, the lamps envelope is filled with a metallic halide, mercury, and a rare gas. The quantities of the materials employed are limited as in the case of the electrode lamp to provide a total vapor pressure at its operating temperature of less than 760 torr. An energizing coil is positioned contiguous to the envelope thereby to inductively excite the vapor in the envelope. By limiting the power applied to the vapor, as well as the vapor pressure, the desired spectral emission having reduced spectral line broadening and shifting is obtained.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings in which:

FIGURE 1 is a partial block and partial schematic diagram of an atomic spectral light source constructed in accordance with one embodiment of the invention; and

FIGURE 2 is a partial block and partial schematic diagram of an atomic spectral light source constructed in accordance with another embodiment of this invention.

The light source of this invention, as shown in FIG. 1 comprises an enclosed, generally cylindrical, transparent envelope 2 such as quartz within which two small electrodes 4, 6 are mounted by means of lead wires 8, 10 sealed in opposite ends of the cylindrical envelope 2. The electrodes 4, 6 and lead wires 8, 10 may be tungsten wire or other suitable material. The electrodes 4, 6 are wound to be substantially identical in size and shape and areformed to be capable of providing a relatively large source a of electrons. Typically they may have either an alkaline earth oxide emission mix on them or be formed to have a thorium sliver in their core. The lead wires 8, provide mechanical and electrical connections between the electrodes and the exterior of the envelop 2. The lead wires 8, 10 extend outside of the envelope 2 land are connected across a suitable high voltage power supply 12 (either A.C. or DO). A current limiting means such as a ballast resistor 14 is connected in series with the power 12 thereby to limit the current flow between the electrodes 4, 6. The ballast resistor 14 is selected to limit the current flow to a desired value which is a function of the spectral characteristics as will be described hereinafter. The power supply volt-age in accordance with known techniques is selected to be high enough to initiate a discharge by the electrodes 4, 6. Typically the power supply is in the order of sever-a1 hundred volts.

The envelope 2 is evacuated and desirably, to achieve stability and long life, is thoroughly outgassed under high vacuum conditions to substantially eliminate the emission of gaseous particles from the surfaces of the electrodes and the envelope during subsequent operation. With suitable outgassing the usage of a getter is unnecessary. Next a halogen combined with the metal whose spectral lines it is desired to obtain is introduced into the tube as a salt, as described in the Reiling article. In a preferred embodiment a metallic iodide is employed. For example, if aluminum spectral lines are desired, a measured amount of aluminum iodide is introduced into the tube.

The precise amount of metal salt added will depend on the particular metal-halogen used, the volume of the tube envelope 2, the degree of pressure broadening of the spectral lines that can be tolerated, and the brilliance required of the spectral lines. After the metallic halogen is introduced a measured amount of mercury, which comple-tely evaporates at normal operating temeperatures of the arc, is added to the envelope 2. The total operating vapor pressure of the lamp is controlled by varying the amount of mercury added. As a necessary condition to start a discharge in the lamp, an inert gas or preferably one of the noble gases such as argon is added to the envelope just :before sealing olf. Typically the noble gas is added at a pressure of less than millimeters of mercury although the precise pressure used will vary with the metallic halogen employed. The amount of mercury added is adjusted to provide a total vapor pressure within the envelope 2 including the vapor pressure of the metallic iodide and the pressure of the inert gas of less than one atmosphere when operated.

In a particular lamp from which spectral lines of aluminum are desired, a typical quartz (fused silica) envelope may be selected 4 centimeters (cm.) long and 1 cm. in diameter Within Which the electrodes are separated by 3 cm. With these dimensions, the internal volume of the envelope approximates 3 cubic centimeters. Into this envelope after outgassing is placed 3 milligrams (mg.) of mercury and 2 mg. aluminum iodide (AlI Immediately prior to seal off, 5 torr of argon is inserted. Under typical operating conditions a source of potential of 1,000 volts and a 2,000-ohm ballast resistor is employed to provide a lamp temperature of approximately 500 C. In this instance the total pressure within the lamp approximates 32 torr. Quite obviously, other operating temperatures may be employed as desired, in which case, the potential source, ballast resistor and quantities of mercury and aluminum iodide will vary.

Now with the application of voltage across the electrodes 4, 6 a luminous discharge is established and the resulting high temperatures caused by the electrical discharge vaporizes the mercury and the metallic iodide. Electron and ion collisions with the metal atoms of the metallic iodide raise the metal to an excited state, and in the process of returning it to ground state the desired atomic metal spectral lines are emitted. By restricting the electrical discharge as Well as the pressure of the metal iodide in accordance with this invention, the spectral emission occurs without appreciable line broadening or shifting. The particular ballast resistor employed to limit the current will depend upon the particular metal, the particular halogen, as well as the particular inert gas with which the envelope is filled. The various combinations possible are too numerous to specify with particularity.

Such lamp is particularly useful for such spectroscopic purposes as atomic absorption spectroscopy, absorption spectroscopy, as well as use as a spectroscopic standard. The improved light source thus produced is relatively economic and merely by varying the metallic iodide, various spectral lamps are available. Such light source would for example permit the accurate and rapid analysis of trace metals such as sodium, potassium, calcium, magnesium, and zinc by normal spectroscopic techniques. This invention is only limited to the use of those metals from which a halogen salt may be formed.

In the manufacture of this tube conventional outgassing techniques may be employed. One such known method requires that the envelope be evacuated and internally torched several times by a gas flame. Evacuation is accomplished by any suitable mechanical pump. Subsequent to each torching the evacuation process is continued until a satisfactory vacuum is obtained. An electrical potential on the order of several hundred volts is then applied to one electrode with the other connected to ground, and spectroscopically pure argon or other noble gas such as neon allowed to enter slowly into the envelope. The pressure of the argon is slowly increased until an electrical discharge is obtained. The discharge is permitted to continue for a period of time at the high est current possible without melting the cathode. The power supply is then cut off and the tube rapidly evacuated with the cathode still hot. At a particular pressure which is dependent upon the tube geometry and the particular cathode metal, sputtering occurs which aids in cleaning the electrode surfaces. This process is repeated several times for each electrode after which the tube is filled with the metallic iodide, mercury, and inert gas and sealed off. Although the lamp of this invention has been described using a metallic iodide, it should be recognized that most halogens may often be combined with the de sired metal.

Referring now to FIG. 2 there is shown another embodiment of this invention in which an electrodeless discharge lamp is constructed. There is shown a generally cylindrical, transparent sealed envelope 20 made from the same materials as used in the lamp of FIG. 1. Surrounding the envelope 20 is an energizing coil 22 which is supplied by radio frequency power from a source 24. The source 24 is preferably an R.-F. oscillator the output power level of which is made variable by well-known techniques. The energizing coil 22 need not surround the envelope 20, but may be adjacent or contiguous to the envelope and so positioned as to direct its R.-F. power into the gaseous discharge region within the envelope.

The interior of the envelope 20 is filled, in the same manner as described in conjunction with the filling of the envelope 2 of FIG. 1, with a metallic halide salt, mercury, and an inert or rare gas. The quantities employed are substantially the same as those described in conjunction with the embodiment of FIG. 1. The region within the envelope 20 when energized by suitable power sources provides spectral radiation characteristic of the metal halide employed and may be termed an electrodeless discharge region.

In its operation, by limiting the power supplied to the discharge region within the envelope 20, spectral emission characteristics are obtained that are relatively free from line broadening and shifting effects that have normally been encountered in optical sources of the prior art. This freedom from spectral line shift and broadening is v.3 particularly enhanced by limiting the total operating pressure of the combined vapors within the envelope to below one atmosphere (760 torr).

The optical radiation produced by the discharge may be directed out through the envelope 2 between the turns of the coil 22. Alternatively the optical radiation may be directed through the end of the envelope 20. Typically the R.-F. source frequencies employed would be in the range of 1 megacycle to 2,000 megacycles, although higher frequencies may be employed for particular applications as desired.

There has thus been described an improved light source which provides relatively sharp and stable spectral lines corresponding to various atomic metals.

It will be obvious that various modifications may be made in the apparatus and in the manner of operating it. It is intended to cover such modifications and changes as would occur to those skilled in the art, as far as the following claims permit and as far as consistent with the state of the prior art.

What is claimed is:

1. Apparatus for providing a spectral light source comprising:

a sealed quartz envelope;

a pair of electrodes disposed within said envelope;

lead wires supporting said electrodes and embedded in and extending through said envelope;

an atmosphere within the envelope having a total pressure at the operating temperature of the light source of less than 760 torr including a metallic halogen for emitting spectral lines characteristic of the metal, mercury vapor, and an inert gas;

and means adapted to connect a source of potential across said electrodes for establishing an are therebetween.

2. Apparatus for providing a spectral light source comprising:

a sealed envelope;

a pair of electrodes disposed within said envelope;

lead wires supporting said electrodes and embedded in and extending through said envelope; an atmosphere within the envelope having a total vapor pressure at the operating temperature of the light source of less than 760 torr including a metallic iodide for emitting spectral lines characteristic of the metal, mercury vapor, and a noble gas; means adapted to connect a source of potential across said electrodes for establishing an arc therebetween;

and means for limiting the arc current to a sulficiently low value to prevent appreciable spectral line broadening and shifting.

3. Apparatus for providing an atomic spectral light source comprising:

a sealed transparent envelope;

a pair of electrodes disposed within said envelope;

lead wires supporting said electrodes and embedded in and extending through said envelope; an atmosphere within the envelope having a total vapor pressure at the operating temperature of the light source of less than 760 torr, said atmosphere including metallic iodide vapor for emitting spectral lines characteristic of the metal, mercury vapor, and argon gas; means adapted to connect a source of potential across said electrodes for establishing an arc therebetween;

and means including a ballast resistor connected in series with one of said electrodes for limiting the arc current to a sufficiently low value to prevent appreciable spectral line broadening and shifting.

4. Apparatus for providing an atomic spectral light source comprising:

a sealed transparent envelope;

a pair of electrodes disposed within said envelope at opposite sides thereof;

lead wires supporting said electrodes and embedded in and extending through said envelope;

an atmosphere within the envelope having a total vapor pressure at the operating temperature of the light source of less than 760 torr, said atmosphere including a metallic iodide for emitting spectral lines characteristic of the metal, mercury vapor, and an inert gas; and

means adapted to connect a source of potential across said electrodes for establishing an arc therebetween.

5. A spectral light source comprising:

a sealed envelope;

' a pair of electrodes disposed within said envelope;

7 lead wires supporting said electrodes and embedded in and extending through said envelope;

a metallic halogen compound disposed within said envelope for providing atomic metal adapted to emit characteristic spectral lines when excited, and mercury disposed within said envelope for limiting the mean free path of the metallic atoms, the quantities of said metallic halogen and said mercury being such as to provide a vapor pressure of less than one atmosphere at the operating temperature of the light source.

6. A spectral light source comprising:

a sealed envelope;

a pair of electrodes disposed within said envelope;

lead wires supporting said electrodes and embedded in and extending through said envelope;

a metallic halogen compound disposed within said envelope for providing atomic metal adapted to emit characteristic spectral lines when excited, and mer cury disposed within said envelope for limiting the mean free path of the metallic atoms, the quantitiies of said metallic halogen and said mercury being such as to provide a vapor pressure of less than one atmosphere at the operating temperature of the light source;

and a noble gas for initiating an electrical discharge between the electrodes.

7. A spectral light source comprising:

a sealed envelope;

a pair of electrodes disposed within said envelope;

lead wires supporting said electrodes and embedded in and extending throngh said envelope;

a metallic halogen compound disposed within said envelope for providing atomic metal adapted to emit characteristic spectral lines when excited, and merc-ury disposed within said envelope for limiting the mean free path of the metallic atoms, the quantities of said metallic halogen and said mercury being such as to provide a vapor pressure of less than one atmosphere at the operating temperature of the light source;

and a noble gas whose component of pressure is less than 20 torr for initiating an electrical discharge between the electrodes disposed within said envelope at opposite sides thereof.

8. The light source set forth in claim 7 wherein said noble gas is argon.

9. Apparatus for providing a spectral light source comprising:

a gaseous discharge region containing a metallic halogen compound for emitting spectral lines characteristic of the metal, mercury vapor, and an inert gas, said discharge region having a total pressure at its operating temperature of less than 760 torr,

and energizing means for establishing a gaseous discharge within said region.

10. The apparatus set forth in claim 9 which also includes means associated with said energizing means for limiting the energy supplied said discharge region to a sufiiciently low value to prevent appreciable spectral line broadening and shifting.

11. The apparatus set forth in claim 9 wherein said energizing means comprises an electromagnetic means adjacent said discharge region for exciting a discharge therein.

12. The apparatus set forth in claim 11 which also includes means associated With said energizing means for limiting the energy supplied said discharge region to a suificiently low value to prevent appreciable spectral line broadening and shifting.

13. The apparatus set forth in claim 9 wherein said energizing means includes means for establisihng an arc in said discharge region.

14. The apparatus set forth in claim 13 which also includes means associated with said energizing means for limiting the energy supplied said discharge region to a sufficiently low value to prevent appreciable spectral line broadening and shifting.

References Cited by the Examiner UNITED STATES PATENTS Steinmetz 313220 X Beese 313223 X Beese et al 313--223 X Jaumann et al. 313 185 X Bauer 313-225 X Reiling 313229 X 10 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

3. APPARATUS FOR PROVIDING AN ATOMIC SPECTRAL LIGHT SOURCE COMPRISING: A SEALED TRANSPARENT ENVELOPE; A PAIR OF ELECTRODES DISPOSED WITHIN SAID ENVELOPE; LEAD WIRES SUPPORTING SAID ELECTRODES AND EMBEDDED IN AND EXTENDING THROUGH SAID ENVELOPE; AN ATMOSPHERE WITHIN THE ENVELOPE HAVING A TOTAL VAPOR PRESSURE AT THE OPERATING TEMPERATURE OF THE LIGHT SOURCE OF LESS THAN 760 TORR, SAID ATMOSPHERE INCLUDING METALLIC IODIDE VAPOR FOR EMITTING SPECTRAL LINES CHARACTERISTIC OF THE METAL, MERCURY VAPOR, AND ARGON GAS; MEANS ADAPTED TO CONNECT A SOURCE OF POTENTIAL ACROSS SAID ELECTRODES FOR ESTABLISHING AN ARC THEREBETWEEN; AND MEANS INCLUDING A BALLAST RESISTOR CONNECTED IN SERIES WITH ONE OF SAID ELECTRODES FOR LIMITING THE ARC CURRENT TO A SUFFICIENTLY LOW VALUE TO PREVENT APPRECIABLE SPECTRAL LINE BROADENING AND SHIFTING. 